Diamond heat sink comprising synthetic diamond film

ABSTRACT

A diamond heat sink of the present invention comprises: 
     a support layer consisting of substantially undoped vapor phase synthetic diamond; 
     a heat sensitive layer consisting of doped vapor phase synthetic diamond formed on the surface of the support layer; 
     an insulation layer consisting of substantially undoped vapor phase synthetic diamond formed on a predetermined region of the heat sensitive layer; and 
     an electrode formed on the heat sensitive layer. The electrode typically consists of a metal, preferably Ti/Mo/Au or Ti/Pt/Au. 
     The diamond heat sink of the present invention may further include a highly-doped layer for creating Ohmic contacts with the metal electrode, which is made of the vapor phase synthetic diamond having high impurity levels, and which is disposed between the metal electrode and the heat sensitive layer.

BACKGROUND OF THE INVENTION

1. Filed of the Invention

The present invention is directed to a diamond heat sink having anexcellent heat release characteristics, which incorporates a thermistoror a cooling element with higher thermal response, and is also directedto a method of manufacturing it.

2. Related Background Art

Diamond is a suitable material for heat sink, because diamond hasexcellent thermal conductivity of more than 2,000 [W/m·K]. Further,since diamond can acquire the characteristic of semiconductor materialwhen impurities are doped into the diamond, the resultant doped diamondcan be employed as a thermistor material which has an capabilities forthe use at elevated temperature and has a quick thermal responsecharacteristics.

Japanese Patent Laid-open No. 63-184304 discloses a thermistor made withdiamond. A semiconductor diamond film is formed on a substrate by vaporphase synthesis. The semiconductor diamond film of this reference servesas a heat sensitive film. Ohmic electrodes are formed on the heatsensitive film. The resistivity of the heat sensitive film are measuredthrough these Ohmic electrodes to determine the temperature of the heatsensitive film. Examples of the available substrates on which the heatsensitive film is formed include single crystal diamond substrate, metalhaving a high melting point and a high thermal conductivity,semiconductor, and other compound(s) thereof.

In Japanese Patent Laid-open No. 5-67711 discloses a thermistor made ofdiamond to be integrated with a heat sink made of diamond, and themanufacturing method of the thermistor-integrated heat sink. Accordingto the construction of the thermistor with this manner, thesemiconductor diamond film as the heat sensitive film is formed on apredetermined region of a heat sink material by the vapor phasesynthesis, and metal electrodes are then formed on the heat sensitivefilm. A metallized film is also formed on the surface of the heat sinkmaterial next to the thermistor. The exothermal elements such as diodeand FET are seated onto the metallized film for their temperature to bemeasured. The exotherms from the element on the metallized film isconducted to the thermistor through the heat sink material, and thetemperature of the element can be measured. Japanese Patent Laid-openNo. 5-67711 also shows another thermistor-integrated heat sink. Thesemiconductor diamond film as the heat sensitive film is formed on theentire surface of the heat sink material and metal electrodes are formedonto the heat sensitive film. A metallized film is formed onto thesurface of the heat sensitive film between the metal electrodes, and theexothermal elements are seated onto the metallized film. In both of thethermistor-integrated heat sinks of the Japanese Patent Laid-open No.5-67711, single crystal diamond, metals having high melting point andhigher thermal conductivity (or refractory metals with higher thermalconductivity), semiconductor or other compound(s) thereof are used forthe heat sink material.

As mentioned above, in Japanese Patents Laid-open Nos. 63-184304 and5-67711, the single crystal diamonds are employed as the substrate ofthe thermistor of the former and as the heat sink material of thethermistor-integrated heat sink of the latter, respectively. Singlecrystal diamonds of a consistent quality can be obtained throughartificial synthetic method under extra high pressure, which are stillcostly.

When the vapor phase synthesis is applied to obtain the diamond heatsink, a thicker diamond would be required for the substrate and the heatsink, because of insufficient performances of the diamonds made viavapor phase synthesis. Further, since the vapor phase synthetic diamondis polycrystalline, the surface of the diamond would be considerablyrough. Thus, the rough surface of the synthetic diamond needs to bepolished and made flat to mount the exothermal elements. However, sincediamond is an extremely hard material, the polishing process forsynthetic diamond includes significant difficulties.

In the heat sink construction of Japanese Patent Laid-open No. 5-67711where the thermistor and the exothermal element are arranged in parallelon the heat sink material, the exotherms from the exothermal element istransferred through heat sink material to the thermistor. Thus, the heatwould be detected with some delay, and some of the heat would beabsorbed by the heat sink material in the heat transfer process from theexothermal element to the thermistor. Therefore, thethermistor-integrated heat sinks of the above references haveinsufficient abilities to precisely detect and control a temperature ofthe mounted exothermal element.

U.S. Pat. No. 5,022,928 discloses a film-shaped thermoelectric heat pumpof the Peltier element type in which Bi₂ Te₂ etc. are laminated onto asubstrate. This patent, however, not directed to the improvement of heattransfer. In addition, the patent does not describe the point of quickresponse required when it is used as a heat sink.

Consequently, it is desirable to present a heat sink with a quickresponse which comprises a synthetic diamond made via a vapor phasesynthesis.

SUMMARY OF THE INVENTION

The present invention satisfies the above requirements. A diamond heatsink of the present invention comprises: a support layer consisting ofsubstantially undoped vapor phase synthetic diamond; a heat sensitivelayer consisting of doped vapor phase synthetic diamond formed on thesurface of the support layer; an insulation layer consisting ofsubstantially undoped vapor phase synthetic diamond formed on apredetermined region of the heat sensitive layer; and an electrodeformed on the heat sensitive layer. The electrode typically consists ofa metal, preferably Ti/Mo/Au or Ti/Pt/Au.

The diamond heat sink of the present invention may further include ahighly-doped layer for creating Ohmic contacts with the metal electrode,which is made of the vapor phase synthetic diamond having high impuritylevels, and which is disposed between the metal electrode and the heatsensitive layer.

The diamond heat sink of the present invention may further include ametallized layer formed onto the insulation layer. The metallized layermay be a single layer of Ti/Mo/Au, Ti/Pt/Au or Au with Sn, or multiplelayers thereof.

Each of the above diamond heat sinks may further include a coolingelement formed on the reverse side of the support layer.

The cooling element may be a Peltier element comprising: a first metallayer formed on the reverse side of a support layer; a p-typesemiconductor layer consisting of a diamond doped with a p-type impuritywhich contacts to the first metal layer; an n-type semiconductor layerconsisting of a diamond doped with an n-type impurity which contacts tothe first metal layer; a second metal layer which contacts to the p-typesemiconductor layer; and a third metal layer which contacts to then-type semiconductor layer. The Peltier element may consist of pluralityof II-shaped submodules, each of which is connected in series to form aPeltier module.

The method for manufacturing the diamond heat sink according to thepresent invention comprises the following steps of: forming asubstantially undoped diamond on a substrate via vapor phase synthesisso that an insulation layer of the undoped diamond is formed on thesubstrate; forming a doped diamond on the insulation layer via vaporphase synthesis so that a heat sensitive layer of semiconductor diamondis formed on the insulation layer; forming a substantially undopeddiamond on the heat sensitive layer via vapor phase synthesis so that asupport layer of undoped diamond is formed on the heat sensitive layer;removing the substrate so that the substantially flat surface of theinsulation layer is exposed; selectively removing the predetermined partof the exposed surface of the insulation layer so that the predeterminedarea of the surface of the heat sensitive layer is further exposed; andforming an electrode on the surface of the exposed heat sensitive layer.

The method for manufacturing the diamond heat sink according to thepresent invention may further comprise the step of forming a metallizedlayer on the other surface of the insulation layer so that themetallized layer is disposed on the insulation layer surface oppositethe heat sensitive layer.

The exotherms from the exothermal element such as diode and FET which ispositioned on the insulation layer is transferred via the insulationlayer to the heat sensitive layer contacting the insulation layer, thenis conducted to the support layer contacting the heat sensitive layer,and is finally released at the support layer. Since each of theinsulation layer, heat sensitive layer and the support layer consists ofdiamond having excellent heat conductivity, the exotherms from theexothermal element on the insulation layer or the metallized layer onthe insulation layer is instantly conducted to the support layer. Inaddition, since the thermal response rate of the thermistor comprisingthe diamond heat sensitive layer is excellent, the resistance of theheat sensitive layer between the electrodes quickly responds to thetemperature of the element on the insulation layer. Since the thermalcapacity of the insulation layer is small, most of the exotherms fromthe exothermal element on the insulation layer is transferred to theheat sensitive layer. The contact resistance between the electrodes andthe heat sensitive layer can be reduced, by means of highly-doped layerdisposed between the electrodes and the heat sensitive layer. Therefore,a heat sink with higher response characteristic can be produced.

A smooth surface of the insulation layer can be obtained without anydifficulties in the manufacturing processes, by means of removing thesubstrate to expose the smooth surface of the insulation layer, which isoriginally formed or deposited on the smooth surface of the substrate.Therefore, the smooth surface of the insulation layer which consists ofthe vapor phase synthetic polycrystal diamond can be obtained andutilized for the surface on which the exothermal element is mounted.Thus, a polishing process can be avoided for the manufacture of a heatsink using vapor phase synthetic diamond.

Further, for another heat sink according to the present invention whichfurther has a metallized layer on the insulation layer, an exothermalelement such as diode and FET is seated on the metallized layer so thatthe exotherms from the exothermal elements is transferred via themetallized layer and the insulation layer.

The surface of the insulation layer of the alternative diamond heat sinkaccording to the present invention may be processed to have surfaceshape corresponding to the shape of the exothermal element to bemounted, so that the exothermal element would fit to the processedsurface of the insulation layer. A metallized layer can further beformed on or covered to the processed insulation layer, having theequivalent shape or pattern of the surface of the insulation layer. Theexotherms from the exothermal element can be sufficiently conducted tothe insulation layer of the alternative heat sinks according to thepresent invention, because the exothermal element can fit to theprocessed surface of the insulation layer or metallized layer on theinsulation layer so that the contacting area between the insulationlayer and the element, or thermal conducting area, is sufficientlylarge. The sufficiently large thermal conducting area would combine withthe sufficient endothermal capacity of the diamond heat sink to giveexcellent heat transfer performances. The alignment for mounting theexothermal element on the processed surface can also be suitably carriedout.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G show a cross sectional view of a diamond heat sinkcontaining a thermistor of a first embodiment, respectively. The figuresillustrates the manufacturing steps of the first embodiment according tothe present invention;

FIG. 2 illustrates a cross sectional view of a diamond heat sinkcontaining a thermistor of a second embodiment according to the presentinvention;

FIGS. 3A-3D show a cross sectional view of a diamond heat sinkcontaining a thermistor and a Peltier element of a third embodiment,respectively. The figures illustrates the manufacturing steps of thethird embodiment according to the present invention;

FIG. 4 also illustrates a cross sectional view of the diamond heat sinkcontaining the thermistor and the Peltier element of the thirdembodiment;

FIGS. 5A-5C show a cross sectional view showing a diamond heat sinkcontaining a thermistor of a fourth embodiment, respectively. Thefigures illustrates the manufacturing steps of the fourth embodimentaccording to the present invention;

FIG. 6 is a perspective view of a modification of the diamond heat sinkof the fourth embodiment;

FIGS. 7A-7C illustrate a cross sectional view of an alternative diamondheat sink containing a thermistor and a Peltier element of a fourthembodiment, respectively; and

FIGS. 8A-8G illustrate a cross sectional view of a diamond heat sinkcontaining a thermistor of the fifth embodiment, respectively. Thefigures illustrates the manufacturing steps of the fifth embodimentaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

Each of the FIG. 1A to 1G is a cross sectional view of a heat sink,showing a manufacturing steps of a diamond heat sink of the firstembodiment of the present invention. The diamond heat sink manufacturedin the following manner.

First, a substantially undoped diamond was deposited on a flat surfaceof a silicon (Si) substrate 1 via vapor phase synthesis to forminsulation layer 2 (see FIG. 1A). The deposition was carried outmicrowave plasma chemical vapor deposition (CVD) using microwave of 2.45GHz, and under the synthesis condition I shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        SYNTHESIS    I          II      III                                           CONDITIONS   (undoped)  (doped) (highly doped)                                ______________________________________                                        hydrogen (sccm)                                                                            200        200     200                                           methane (sccm)                                                                              1          1       1                                            1000 ppm diboran (sccm)                                                                     0         0.02-1   10                                           microwave power (W)                                                                        400        400     400                                           pressure (Torr)                                                                             40         40      40                                           substrate temp. (°C.)                                                               930        930     930                                           ______________________________________                                    

A mixed gas consisting of hydrogen with a flow rate of 200 sccm andmethane with a flow rate of 1 sccm was introduced into the CVD chamberunder the chamber pressure of 40 Torr. The plasma was created in theprocessing chamber by applying 400 W microwave power. The undopeddiamond, thus, deposited on the surface of the Si substrate 1 at thetemperature of 930° C. to form the insulation layer 2. The insulationlayer 2 has the thickness of 5 μm. The level of impurities wasapproximately equal to or less than 10¹⁴ cm⁻³, which is regarded assubstantially undoped.

Next, semiconductor diamond was deposited on the surface of theinsulation layer 2 via microwave plasma CVD to form a heat sensitivelayer 3 (FIG. 1B). The deposition was carried out according to thesynthesis conditions II of Table 1, where boron is doped into thediamond of the layer 3. The mixed gas of a 1000 ppm diboran (flow ratewas from 0.02 to 1 sccm), hydrogen (flow rate of 200 sccm) and methane(flow rate of 1 sccm) was introduced into the CVD chamber to create thechamber pressure of 40 Torr. Then the plasma was created by applying 400W microwave power. Semiconductor diamond was thus deposited on theinsulation layer 2 at the temperature of 930° C., and the heat sensitivelayer 3 was formed on the insulation layer 2. The thickness of the heatsensitive layer 3 was 2 μm, and the level of impurities of the heatsensitive layer was approximately from 10¹⁶ to 10¹⁹ cm⁻³.

Then, a substantially undoped diamond was deposited on the heatsensitive layer 3 via microwave plasma CVD to form a support layer 4(FIG. 1C). The deposition process was carried out according to thesynthesis conditions I of Table 1. The thickness of the support layer 4was up to 100 μm. Since the diamond layer deposited by CVD ispolycrystal, the exposed surface was considerably rough, as shown in theFIG. 1C. The impurity level of the diamond of the support layer 4 wasapproximately equal to or less than 10¹⁴ cm⁻³.

Then, the Si substrate 1 was removed by etching process using themixture of fluoric acid and nitric acid at the ratio of 1:1, and thesurface of the insulation layer 2 opposite the heat sensitive layer 3was thus exposed (FIG. 1D). At this stage, the support layer 4positioned on the upper side is turned upside down to be positioned onthe lower side or bottom, and the insulation layer 2 is then placed onthe upper side or top.

Further, a aluminum mask was formed on the predetermined portion aroundthe center of the exposed surface of the insulation layer 2. Theinsulation layer 2 other than the portion covered with the mask, wasremoved by etching. This diamond etching process was carried out inaccordance with the conditions indicated in the Table 2.

                  TABLE 2                                                         ______________________________________                                        O2/Ar   Flow rate RF power at 1.356 MHz                                                                        etch rate                                    ______________________________________                                        1/99    100 sccm  100 W          15 nm/min.                                   ______________________________________                                    

A mixed gas containing 1% oxygen and 99% argon was introduced into thechamber at a flow rate of 100 sccm and the plasma was made by applyingRF power of 1.356 MHz and 100 W. Then only the uncovered portion of theinsulation layer 2 was thus selectively etched at an etching rate of 15nm/min (FIG. 1E).

Then, a pair of electrodes 5a and 5b were formed onto the exposedsurface of the heat sensitive layer 3, which create an Ohmic contact tothe electrodes (FIG. 1F). The electrodes were made of Ti/Mo/Au metal.The contact resistance between the electrodes 5a, 5b and the heatsensitive layer 3 was sufficiently low. It was found according to thefurther investigation that the sufficiently low contact resistance wasmaintained to be substantially constant at the elevated temperature.

Finally, a copper support table 6 is fixed over the rough surface of thesupport layer 4 via silver solder 7 (FIG. 1g).

A heat from the exothermal element such as semiconductor laser diode wasplaced on the insulation layer 2. The exotherms from the element on theinsulation layer 2 is transferee by conduction via insulation layer 2 tothe heat sensitive layer 3. The heat was then transferred to the supportlayer 4 and the copper support table 6. Since the insulation layer 2,heat sensitive layer 3 and the support layer 4 are made of diamondhaving excellent heat conductivity, the heat from the element on theinsulation layer 2 would be quickly transferred by conduction to thesupport layer 4. The heat would be then quickly released from the heatsink consisting of the support layer 4 and the copper support table 6.Since the thermal response rate of the thermistor is high, theresistance between the electrodes 5a and 5b and the heat sensitive layer3 promptly varies to the resistance corresponding to temperature of theelement positioned on the insulation layer 2. Since the thermal capacityof the insulation layer 2 is small, most of the heat from the exothermalelement on the insulation layer 2 would be transferred to the heatsensitive layer 3. Therefore, the temperature of the element on theinsulation layer 2 is highly precisely and quickly measured by applyingelectrical current between the electrodes 5a and 5b and detectingresistance value across the heat sensitive layer 3. At the same time,the exotherms from the element is conducted to the support layer 4 andthe support table 6 and is then quickly released.

In the manufacturing process of the diamond heat sink of thisembodiment, the synthetic diamond was deposited onto the flat and evensurface of the substrate to form the insulation layer 2. And the Sisubstrate 1 was then removed and the smooth surface of the insulationlayer 2 was exposed. Thus, the smooth surface of the insulation layer 2can be employed as the surface on which the element is mounted, byreversing the position at the step shown in FIG. 1D. Therefore, thesynthetic diamond can be used for manufacturing the thermistor withoutany necessity for polishing the rough surface of vapor phase syntheticdiamond. The manufacturing cost is thus highly reduced.

The further investigation was carried out to determine characteristicsof thermistor by varying doping level of the heat sensitive layer 3. Theresult is shown in Table 3. The variation of the boron level wasaccomplished by varying the concentration of diboran gas in the gasmixture including hydrogen and methane in the synthesis conditions II ofTable 1.

                  TABLE 3                                                         ______________________________________                                        THERMISTOR   I          II       III                                          ______________________________________                                        Boron conc. (ppm)                                                                          20         100      1,000                                        Resistivity at                                                                             7.2 M      240 k    720                                          room temp. (Ω)                                                          Temp. available                                                                            0-800      0-800    0-800                                        for use (°C.)                                                          B constant (K)                                                                             4,990      3,940    2,550                                                     (0.43 eV)  (0.34 eV)                                                                              (0.22 eV)                                    Resistivity changing                                                                       399 k      10.5 k   20.4                                         rate (Ω/°C.)                                                     ______________________________________                                    

The thermistor I was obtained when doped diamond was deposited using 20ppm diboran gas in the gas mixture to form heat sensitive layer 3. Thethermistor II was obtained by using 100 ppm diboran gas, and thethermistor III by using 1,000 ppm diboran gas, respectively.

Since the thermistor and the heat sink is made of diamond, thethermistor can be used at high temperature from 0° C. to 800° C. Thethermistor III is the thermistor of the present embodiment, the standardresistance value of which at room temperature was relatively low (720Ω). The thermistor constant B was 2,550 K and the activation energy is0.22 eV, and thus the changing rate of the thermistor resistance at roomtemperature was 20.4 Ω°C.⁻¹.

The standard resistance value at room temperature would rise as theboron level is lowered. That is, the standard resistance value in thethermistor II using 100 ppm diboran gas was 240 KΩ, and the thermistor Iusing 20 ppm diboran gas was 7.2 MΩ. The range of temperature at whichthe thermistor can be used is fixed from 0° C. to 800° C., regardless ofthe variation of the boron doping level. The thermistor constant B wouldbecome larger as the boron level is lowered. That is, the B constant ofthe thermistor II was 3,940 K and the activation energy was 0.34 eV, andthe constant B of the thermistor I was 4,990 K and the activation energywas 0.43 eV. Thus, the resistance changing rate at room temperaturewould also become larger as the boron level is lowered. The thermistorII which was formed using 100 ppm diboran gas has the changing rate of10.5 KΩ°C⁻¹, and the thermistor I of 20 ppm diboran has the changingrate of 399 KΩ°C.⁻¹.

Therefore, a suitable thermistor can be selected from thermistor I, IIand III, according to the operating conditions.

The time constant for giving the rate of thermal response of thethermistor, in which the heat sensitive layer 3 was formed by using themixed gas at 100 ppm diboran, was measured, and the value of the timeconstant was 0.05 second. This means that the thermistor has an abilityto achieve the temperature change in 63.2% of the temperature difference(t₁ -t₀) in 0.05 second, when stepwise temperature difference to thetemperature t₁ is given to the thermistor from the initial temperatureof t₀.

Example 2

Now, a diamond heat sink of the second embodiment of the presentinvention will be described. FIG. 2 is a cross sectional view showingthe diamond heat sink of the present embodiment.

The diamond heat sink of the second embodiment further has a highlydoped layer 8 for Ohmic contact, which is disposed between the heatsensitive layer 3 and the electrodes 5a and 5b. The highly doped layer 8consists of diamond which includes impurity at higher level. The highlydoped diamond was deposited on the heat sensitive layer 3 by microwaveplasma CVD to form the highly doped layer 8, according to the diamondsynthesis conditions III of Table 1.

After the patterning of the insulation layer 2 by photolithography, asshown in FIG. 1E, a SiO₂ layer was formed on entire surface of thesubstrate 1. The predetermined portion of the SiO₂ layer was selectivelyremoved by etching, and a surface of the heat sensitive layer 3underlying the etched portion was partly exposed. Next, under thesynthesis conditions III in Table 1, the gas containing 1,000 ppmdiboran at the flow rate of 10 sccm as well as hydrogen at 200 sccm flowrate and methane at 1 sccm flow rate was introduced into the CVD chamberunder the pressure of 40 Torr. The plasma was created by applying 2.45GHz and 400 W microwave power, and doped diamond was deposited on thesurface of the substrate by microwave plasma CVD at the temperature of930° C. Such a vapor-phase growth would occur only on the surface of theheat sensitive layer 3 made of semiconducting diamond layer exposed onthe electrode-forming portion, and would not occur on the SiO₂ layer.Thus, the diamond layer including high level of boron was selectivelydeposited on the diamond surface, and the highly doped layer 8 for Ohmiccontact was formed only on the electrode-forming portion as shown inFIG. 2. After the electrodes 5a and 5b were formed on the highly-dopedlayer 8, the SiO₂ layer was removed by dry etching. Then, the coppersupport table 6 was fixed over the support layer 4, and the heat sinkcontaining the thermistor is thus completed.

In the heat sink of this embodiment, since the highly doped layer 8 wasdisposed between the electrodes 5a and 5b and the heat sensitive layer3, the contact resistance of the electrodes 5a and 5b with the heatsensitive layer 3 was effectively reduced. Thus, the standard resistanceof the thermistor was also reduced as shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        THERMISTOR   I          II       III                                          ______________________________________                                        Boron conc. (ppm)                                                                          20         100      1,000                                        Resistivity at                                                                             6.0 M      200 k    700                                          room temp. (Ω)                                                          Temp. available                                                                            0-800      0-800    0-800                                        for use (°C.)                                                          B constant (K)                                                                             4,990      3,940    2,550                                                     (0.43 eV)  (0.34 eV)                                                                              (0.22 eV)                                    Resistivity changing                                                                       399 k      10.5 k   20.4                                         rate (Ω/°C.)                                                     ______________________________________                                    

The Table 4 shows the characteristics of the different thermistors ofdifferent boron doping levels layer as shown in Table 3. The thermistorIII shown in table 4 corresponds to the thermistor of the secondembodiment, and its standard resistance value at room temperature was aslow as 700 Ω. The standard resistance value of the thermistorcharacteristic II at 100 ppm diboran is 200 KΩ, and the standardresistance value of the thermistor characteristic 1 at 20 ppm diboran is6.0 MΩ. All of the standard resistance values at room temperature werelower than the standard resistance value shown in Table 3.

The range of temperature at which the thermistor can be used, the Bconstant, and the changing rate of resistance are the same as those inTable 3.

Example 3

The heat sink of the present embodiment comprises a Peltier element aswell as a thermistor. FIG. 3A to 3D illustrate the manufacturing processof the heat sink which comprises the thermistor and the Peltier element.

It is known that the thermoelectric power of a semiconductor material,which is the inverse of the Peltier Effect, is proportional to the bandgap of the semiconductor material ("ELECTRONS IN SOLIDS--An introductorySurvey Second Edition", Richard H. Bube, pp.191-192, which is herebyincorporated by reference).

The reference also teaches that excellent thermal conductivity ofdiamond would have affect creating temperature difference in the Peltierelement. However, the excellent Peltier Effect of the diamond Peltierelement would compensate the disadvantage of difficulty on creatingtemperature difference, and therefore the diamond Peltier element cansuccessfully transfer the exotherms from the exothermal element.

The insulation layer 2 made of undoped vapor phase synthetic diamond,the heat sensitive layer 3 of semiconducting vapor phase syntheticdiamond, and the support layer 4 of undoped vapor phase syntheticdiamond were formed on the Si substrate i in order, as in themanufacturing process of the first embodiment (FIG. 1A, 1B and 1C).

Next, a metal layer was deposited on the rough surface of the supportlayer 4 and the patterning was made on the surface of the metal layer. Afirst metal layer 11 is selectively formed on the support layer 4 (FIG.3A). Then, a p-type semiconductor layer 12 and an n-type semiconductorlayer 13 was selectively deposited on the first metal layer 11 byselective CVD method, and the each of semiconductor layers 12 and 13 hasa contact to the first metal layer 11 at one end (FIG. 3B). Thesemiconductor layers 12 and 13 consists of semiconductor materials ofII-VI compound such as bismuth-tellurium semiconductor. An insulationlayer 16 was embedded between the p-type semiconductor layer 12 and then-type semiconductor layer 13, in order to electrically insulate andisolate each of the layers 12 and 13. Then, another metal layer wasdeposited on the entire surface of the substrate, and the patterning wasmade on the surface of the metal layer. A second metal layer 14 and athird metal layer 15 were selectively formed to make contact with thep-type semiconductor layer 12 and n-type semiconductor 13 at the otherend, respectively (FIG. 3C). The Peltier element of the presentembodiment comprises II-shaped submodule units which consists of thefirst metal 11, the p-type semiconductor layer 12, the n-typesemiconductor layer 13, the second metal layer 14 and the third metallayer 15. Each of the submodules are connected in series, and the moduleof the Peltier element is composed of the submodules connected inseries. Then, the Si substrate 1 was removed by etching using the mixedliquid of fluoric acid and nitric acid at the ratio of 1:1, to obtain asmooth surface of the insulation layer 2. The insulation layer 2 is thenreversed upside down, and the smooth surface of the insulator layer ispositioned on the upper face (FIG. 3D).

The patterning was then made on the exposed surface of the insulationlayer 2 as in the first embodiment, and the electrodes 5a and 5b wereformed in contact with the exposed heat sensitive layer 3 (FIG. 1E and1F). As a result, an excellent diamond heat sink containing thethermistor and the Peltier element shown in FIG. 4 was obtained.

As shown in the FIG. 4, an exothermal element 21 to be measured wasplaced on the surface of the insulation layer 2. A power source 22 wasconnected between terminals of the Peltier element, and the electricalcurrent was applied in each of the submodules connected in series.Endotherms was caused in the upper face of the Peltier element, whichcomprises the contact portion between the p-type semiconductor layer 12and the first metal layer 11 and the contact portion between the n-typesemiconductor layer 13 and the first metal layer 11, when electricalcurrent was applied. Exotherms was also caused in the lower face of thePeltier element, which comprises the contact portion between p-typesemiconductor layer 12 and the second metal layer 14 and the contactportion between the n-type semiconductor layer 13 and the third metallayer 15. Consequently, since the Peltier element as cooling element wasformed on the reversed side of the support layer 4, the exotherms fromthe exothermal element was promptly transferred by conduction to thesupport layer 4, and quickly released by the Peltier element.

The exotherms from the exothermal element 21 positioned on theinsulation layer 2 was promptly transferred through the insulation layer2 and the heat sensitive layer 3 to the support layer 4 as in the firstembodiment. The heat reached to the support layer 4 would be promptlyabsorbed into the upper face of the Peltier element, and then releasedinto the atmosphere at the lower face of the Peltier element. Asdescribed earlier in the description on the first embodiment, thetemperature of the exothermal element 21 was instantly and preciselydetected by the thermistor which consists of the heat sensitive layer 3and the electrodes 5a and 5b. Thus, the heat absorption and the heatreleasing by the Peltier element can be precisely and simultaneouslycontrolled by controlling the electrical current applied to the Peltierelement to the level corresponding to the measured element temperature.The temperature of the element 21 placed on the insulation layer 2 canthus be precisely set to the desired temperature without considerabledelay.

The heat sink of the present embodiment is very useful for the use ofcontrolling the operation of electrical devices. For example, when theexothermal element(device) 21 is a semiconductor laser diode (LD), thetemperature of the LD 21 is simultaneously detected by the thermistor athigh thermal response rate, and is quickly cooled to stabilizetemperature of the LD 21 by the Peltier element. As a result, stableoperation of the laser diode was achieved to emit a light with aconstant light strength and a constant wavelength.

In the manufacturing method of the diamond heat sink of the thirdembodiment, the support layer 4 was first formed on the heat sensitivelayer 3, the Peltier element was formed on the support layer 4, then Sisubstrate 1 was removed, and eventually the flat, even and smoothsurface of the insulation layer 2 was exposed. Therefore, a smoothsurface of the synthetic diamond can be obtained for being mounted aexothermal element without difficult polishing process. The heat sinkcontaining the thermistor and the Peltier element can be thus obtainedat low cost.

In the third embodiment, the electrodes 5a and 5b directly contact thesurface of the heat sensitive layer 3. However, a highly doped layer forforming Ohmic contacts may be disposed between the heat sensitive layer3 and the electrodes 5a and 5b as in the second embodiment. In thiscase, the contact resistance of the electrodes 5a and 5b can bemaintained to be low as in the second embodiment.

Example 4

FIG. 5A to 5C shows a cross sectional view of the diamond heat sink ofthis embodiment. A heat sink shown in FIG. 5A is the heat sinkcontaining a thermistor obtained in the first embodiment. In the fourthembodiment, as shown in FIG. 5B, the surface of the insulation layer 2was further processed to fit the shape of an exothermal element to bemounted 31. The exothermal elements 31 was mounted on the processedinsulation layer 2 and the exothermal element could fit to the processedsurface of the insulation layer, as shown in the FIG. 5C. Therefore, thecontacting area between the insulation layer 2a and the element 31 wasmade as large as possible. Since the heat transfer area between theelement 31 and the insulation layer 2a is sufficiently large, theexotherms from the element 31 would be more efficiently transferred tothe insulation layer 2a. The heat from the element 31 was thustransferred to the support layer 4 in shorter time, and the heat releaseeffect of the heat sink was more enhanced.

When the element 31 is, for example, a LD (laser diode), most of theexotherms are from the active layer of the laser diode. The exothermsfrom the LD can be regarded as a exotherm from a point source, asdescribed in the reference; page 442 to page 447: "Journal of LIGHTWAVETECHNOLOGY vol. 11 No. 3 March 1993".

Theoretical analysis of heat release was carried out by a computingsimulation assuming a LD as a point heat source. It was confirmed by thetheoretical analysis that the heat release effect was highly improved,when the surface of the insulation layer 2a was processed to fit theexothermal element 31 and the distance between the active layer of theLD and the surface of the insulation layer 2a was shortened as much aspossible. Experiments using the LD as the element 31 were alsoconducted, and the threshold current for thermorunaway of the LD whichwas mounted on the processed surface of the insulation layer was 50%larger than the case where the surface of the insulation layer 2 wasflat and not processed.

According to the fourth embodiment, the alignment for mounting theelement 31 on the insulation layer 2a was facilitated by processing thesurface of the insulation 2a corresponding to the shape of the element31. For example, when the element 31 is a LD, the LD and optical fibersconnected to the LD can be easily positioned and fixed by wedge-shapedgrooves. That is, as shown in FIG. 6, LD chips 33 and optical fibers 34having wedge-shaped contact portions are aligned, positioned and fixedby a plurality of grooves 32 having wedge-shaped contact portions on thesurface of the insulation layer 2. Thus, in this embodiment, the elementcan be easily mounted and the assembly can be facilitated.

The diamond heat sink described above comprises only a thermistor.However, as in the third embodiment, the heat sink may include both thethermistor and a Peltier element. In this case, the heat sink of thefourth embodiment can attain the equivalent effects as obtained in thethird embodiment, as well as the effects in the fourth embodimentdescribed above. Further, in the heat sink of the fourth embodiment, theelectrodes 5a and 5b directly contact to the heat sensitive layer 3.However, as in the second embodiment, a highly doped layer for creatingOhmic contacts may be disposed between the electrodes 5a and 5b and theheat sensitive layer 3. In this case, the contact resistance of theelectrodes 5a and 5b can be reduced as in the second embodiment, as wellas attaining the equivalent effects obtained in the fourth embodiment.

FIGS. 7A to 7C illustrate alternative heat sink according to the presentembodiment. The heat sink according to the present invention may furthercomprise metallized layer, which is formed on or covered over theinsulation layer 2a, as shown in FIG. 7B. The surface of the insulationlayer 2 of the heat sink shown in FIG. 7A, which is absolutely the sameas one shown in FIG. 5A, was processed and then metal layer of Ti/Pt/Mowas formed onto the processed surface of the insulation layer (FIG. 7B).Then the exothermal elements 31 were placed onto the wedge-shapedsurface of the metallized layer, as shown in FIG. 7C. The metallizedlayer may consist of Ti/Mo/Au or of gold-tin. As shown in FIG. 7C, theexothermal elements 31 suitably fitted to the wedge-shaped surface ofthe metallized layer, resulting in the improvement on the heat transferand the alignment, as described in the previous example of the presentembodiment.

Example 5

In the fifth embodiment, a heat sink having thermistor in the oppositeside to the surface for placing exothermal element was prepared by thesame steps as described in the fourth embodiment, other than the stepsin which the surface of the insulation layer is metallized with Ti/Pt/Auand gold-tin in order. A part of the steps in the fifth embodiment isshown in FIGS. 8A to 8G.

The heat sensitive layer of the doped diamond 3 was deposited on theinsulation layer of the undoped diamond 2 (FIG. 8A). The substrate wasthen turned upside down, and the metal layer 20 consisting of Ti/Pt/Auwas formed on the upper surface and the side edge of the insulationlayer (FIG. 8B). The metal layer of Ti/Pt/Au was formed as anintermediate layer between a diamond and the gold-tin layer, for thepurpose of improvement on the adhesion of diamond with gold-tin. Thesubstrate was again turned around to recover in the initial position,and the electrode was formed on the exposed surface of the heatsensitive layer 3 (FIG. 8C). An aluminum layer 22 for etch masking wasformed on the electrode 5 (FIG. 8C), and the unmasked portion of theheat sensitive layer 3 was then removed by reactive ion etching (FIG.8D). The upper surface of the insulation layer 2 was also coated withthe Ti/Pt/Au, and the entire Ti/Pt/Au layer 20 was covered with gold-tinlayer 24 (FIG. 8F).

The heat sink have a thermistor, which consists of insulation layer 2,heat sensitive layer 3 and electrode 5. The exothermal element should beplaced on the metallized surface opposite the thermistor (FIG. 8G).

In place of inserting the Ti/Pt/Au layer between diamond and gold-tin,the insulation layer 2 can be metallized with a Ti/Mo/Au layer.

As mentioned before, according to the present invention, the exothermsfrom the element mounted on the insulation layer is transferred throughthe insulation layer to the heat sensitive layer. The heat is thentransferred to the support layer, and is finally released to theatmosphere at the support layer. Since the insulation layer, the heatsensitive layer and the support layer are made of diamond havingexcellent heat conductivity, the heat from the element on the insulationlayer can promptly be transferred to the support layer. Also, since thethermal response rate of the thermistor composed of the heat sensitivelayer made of diamond is high, the resistance of the heat sensitivelayer between the electrodes instantly becomes the value responsive tothe temperature of the element mounted on the insulation layer.Moreover, since the thermal capacity of the insulation layer is small,most of the heat from the element placed on the insulation layer istransferred to the heat sensitive layer. Thus, the temperature of theelement on the insulation layer is highly precisely and simultaneouslymeasured, allowing precise controlling of the operation of the mountedelement.

Furthermore, according to the present invention, diamond which is formedor deposited by vapor phase synthetic process such as CVD can be used asthe heat sink substrate without surface polishing process, and thus themanufacturing cost is considerably reduced.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

The basic Japanese Application No. 182380/1994 (6-182380) filed onAugust 3 is hereby incorporated by reference.

What is claimed is:
 1. A diamond heat sink comprising:a support layerconsisting of substantially undoped diamond; a heat sensitive layerconsisting of doped diamond, disposed on the surface of said supportlayer; an insulation layer consisting of substantially undoped diamond,disposed on a predetermined region in the surface of said heat sensitivelayer; and electrodes disposed on said heat sensitive layer, wherein anexothermal element is placed on the surface of said insulation layer,and wherein said heat sensitive layer and said electrodes form athermistor, the electrical resistivity of said thermistor being capableof varying corresponding to heat generated from the exothermal elementand transferred through said insulation layer to said thermistor.
 2. Adiamond heat sink according to claim 1, wherein said undoped and dopeddiamond are deposited by vapor phase synthesis.
 3. A diamond heat sinkaccording to claim 1, wherein said vapor phase synthesis is chemicalvapor deposition.
 4. A diamond heat sink according to claim 1, wherein alevel of impurities included in said doped diamond is in the range of10¹⁶ -10¹⁹ cm⁻³, and wherein a level of impurities of B, P, As or Liincluded in said substantially undoped diamond is equal to or less than10¹⁵ cm⁻³.
 5. A diamond heat sink according to claim 1, which furthercomprises a highly-doped layer for creating Ohmic contact, whichconsists of doped diamond including a impurity at a high concentration,and disposed between said electrodes and said heat sensitive layer.
 6. Adiamond heat sink according to claim 1, wherein a cooling element isdisposed on the surface of said support layer opposite said heatsensitive layer.
 7. A diamond heat sink according to claim 1, whichfurther comprises a metallized layer formed on the surface of saidinsulation layer.
 8. A diamond heat sink according to claim 7, whereinsaid metallized layer consists of a single layer of Ti/Pt/Au, Ti/Mo/Auor Au with Sn, or multiple layers thereof.
 9. A diamond heat sinkaccording claim 6, wherein said cooling element is a Peltier elementconsisting ofa first metal layer formed on the surface of said supportlayer; a p-type semiconductor layer consisting of a diamond doped withp-type impurity, which contacts to said first metal layer; an n-typesemiconductor layer consisting of a diamond doped with n-type impurity,which contacts to said first metal layer; a second metal layer whichcontacts to said p-type semiconductor layer; and a third metal layerwhich contacts to said n-type semiconductor layer, wherein said Peltierelement consists of II-shaped submodules, each of which is connected inseries.
 10. A diamond heat sink according to claim 1, wherein thesurface of said insulation layer is processed to have the shapecorresponding to the shape of the element to be mounted so that theexothermal element fits to processed surface of said insulation layer.11. A diamond heat sink according to claim 10, which further comprises ametallized layer disposed on said processed surface of said insulationlayer so that the exothermal element fits to surface of said metallizedlayer.
 12. A diamond heat sink comprising:a insulation layer consistingof substantially undoped diamond; a thermistor formed on a surface ofsaid insulation layer, said thermistor consisting of a heat sensitivelayer consisting of doped diamond and an electrode formed on said heatsensitive layer; and a metallized layer formed on exposed surface ofsaid insulation layer, wherein an exothermal element is placed on thesurface of said metallized layer opposite said thermistor.